Telephone system



Feb.Y 25, 1941. R a. BUCHANAN Erm.

' TELEPHONE ss'rzu Filed Aug. 16. 1939 6 Sheets-Sheet 1 /NVENroRs R. 8. BUCHA NA N By C .0. KOE CHL/NG MTF/WWW ATTORNEY Luv Fd" 251 1941- R B. BUCHANAN Erm. 2.233.282

' TELEPHONE ss'rnu Filed Aug. 16, 1939 SShnts-Sheet 2 RCI INI/wraps im www r, n N

w www Arr @New Feb. 25, 1941.

6 Sheets-Sheet 3 /N vE/v'ToRs R. B. BUCHANAN anfrage/MN@ A TTORNEV Feb. 25,4 1941 R a. BucHANAN Erm. 2.233.282

'rnLsrn'oNn' ss'rnl Filed Aug. 1e, 1939 6 sheets-sheet 4 unna/NAL ATTORNEY l Feb. 25, 1941. R B. BUCHANAN Erm. 2,233,282

TELEPHONE SYSTB( Filed Aug. 16, 1939 6 Sheets-Sheet 5 v1/wraps RBBUCHANAA/ C .0. K 0E CHL/NG WMM WT M ATT'og/VEV Feb. 25, 1941.

R B. BUCHANAN ET AL TELEPHONE SYSTEM Filed Aug. 16, 1939 /Paumzfa 6 Sheets-Sheet 6 INVENTORS RUCHA/N C QKECHL//VG Patented Feb. 25, 1941 TELEPHONE SYSTEM Russell B. Buchanan, Lynbrook, and Charles D. Koechling, Floral Park, N. Y., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a. corporation of New York Application August 16, 1939, Serial No. 290,366

12 Claims.

This invention relates to signaling systems and particularly to systems comprising automatic switches for establishing signaling connections.

It is the object of the invention to decrease the probability of circuit failure and to increase the reliability of the service rendered by automatic signaling systems.

In telephone systems comprising small dial ofces interconnected by two-way trunks, it is usual l0 to arrange lthe two-way trunk circuits at .the ends of a two-way trunk so that seizure of the trunk at either end effects the marking oi the trunk as busy at both ends. provide one fuse for each oi.' these trunk circuits and to provide a 'relay which operates when the fuse blows for marking the trunk circuit as busy in the banks of the selector switches having access to the trunk .to prevent seizure of the trunk circuit on an outgoing call while the trunk circuit 2 is inoperative.

an alarm is operated when the trunk circuit fuse blows, to summon a. maintenance attendant. If the oiiice is an unattended oiiicc, an alarm call is transmitted over an idle and operative trunk to u an operator in a master oiiice.

In a system comprising two unattended dial ofiices and a master oiiice and in which calls to the master cnice from one of the dial offices are routed through the other dial oiiice, a trunk between the dial oilices may be rendered useless by the failure of the fuse in the .trunk circuit at either end of .the trunk. In such a system the blowing of the fuse in a trunk circuit at the tandem dial oiiice end of a trunk between the dial offices will entirely prevent a call to the master oilice from subscribers in the other dial office if the disabled trunk is first choice .to .the selectors in the other dial oiiice unless means are provided for preventing the selection of the disabled trunk. o This invention is a two-way trunk circuit in which a relay is provided for simulating seizure by a selector when the circuit is rendered inoperative by the blowing of a fuse which is individual to Ithe 5 circuit. l

A feature of the invention is a two-way trunk circuit in which a line relay is responsive to seizure of the trunk at the distant end and a makebusy relay is provided for marking the trunk cir- '50 cuit busy in the banks 'of selectors having access thereto when the circuit becomes inoperative due to the blowing of a fuse individual to the circuit, there being another fuse individual to the line relay.

55 A clear and complete understanding of the in- Furthermore, it is usual Ito If the oflice is an attended oiice, v

vention will be facilitated by considering a telephone system in which the invention is embodied, one such system beingrepresented schematically in the drawings which form a part of this specication. The invention is not limited in its application to the particular system shown in the drawings but is generally applicable to automatic signaling systems.

Referring to the drawings:

Fig. 1 shows a traiiic schematic of an automatic telephone system comprising two dial oiiices and a master cnice;

Fig. 2 shows a two-Way trunk circuit TCI in dial oflice I associated with a trunk TI to a second dial cnice;

Fig. 3 shows a two-way trunk circuit TC2 connected to the trunk TI in the second dial office;

Fig. 4 shows a rotary, non-numerical .trunk selecting switch RS associated with the trunk circuit TCZ of Fig. 3;

Fig. 5 shows a ytwo-way trunk circuit TO3 associated with a trunk T2 between the second dial office and the master oiiice;

Fig. 6 shows a two-way trunk circuit TCl connected to the .trunk T2 in the master office; and

Fig. 7 shows .the positional relation of Figs. 2 to 6, inclusive.

General description The two dial oices and the master omce are each of the common battery type, the apparatus of the various circuit units being connected to the battery .through one or more individual fuses in usual and well-known manner. In accordance with the usual practice, each battery symbol in Fig. 2 represents the same battery in oiiice I; each battery symbol in Figs. 3, 4 and 5 represents the same battery in oiiice 2; and each battery symbol in Fig. 6 representsthe same battery in the master oiiice. In Figs. 2 and 3, the individual fuses for trunk circuits TCI and TC2 are shown, but the fuses for Ithe other circuits are not shown.

The subscribers stations' in oilices I and 2 are of the type which includes a dial or other impulse ent to R. L. Stokely No.;1,799,654, Apri1"1,`1931, for a description of such a switch when employed as a line finder. The line-finder, selector and connector switches are eacl represented by a set of brushes and one or .two sets of terminals, the

associated magnets, relays and other apparatus being omitted.

In response to the origination of a call by a subscriber in ofilce I or omcev 2, an' idle line nder, LFI or LF2, is operated to connect the calling line to a first selector switch, FSI or FS2.

The dialing of the iirst digit directively operates the associated first selector switch to select a desired group of trunks and to automatically select an idle trunk in the selected group. It the called line isin the same oice, the impulses created by the dialing of the succeeding digits of :the called subscribers number directively control a second selector SSI or SS2, and a connector, CI or C2, to complete the connection with a calling line in usual and well-known manner.

All outgoing calls from omce 'I are routed through omceI 2 to an operators position in the master office. By dialing a single digi-t, the first selector FSI is operated to connect the calling l line toa 'two-way trunk circuit TCI, over a trunk TI to a two-way trunk circuit TC2 in ofllce 2.

V'Ihe connection is further extended by the operation of a non-numerical switch RS which :ls individual to trunk circuit TG2 to select an idle two-way trunk circuit TC3 assiated with a trunk T2 leading to the master ofdce. Thevtwoway trunk circuits such as TC3 may also be seized by first selectors FS2 in omce 2 on outgoing calls originated by subscribers in otlice 2. i Each of the trunks T2 terminates at the master oflice in a two-way trunk circuit TC4, seizure of a trunk-T2 in the dial oliice 2 being eiective to operate an answering signal at an operators position. On

a call from a subscriber in ollce I or in oflice 2 f to the master oilice, the connection is under the ljoint control of the calling subscriber and the associated incoming selector IS2. When the opy tion with the called subscribers line.

erator dials the iirst digit, the dial impulses are repeated through trunk circuits TC4 and TC3 to control the group selecting operation of the selector IS2. If the called subscriber is in ofllce 2, .the impulses created by the dialing of the succeeding digits of the called subscribers number effects the operation of a connector C2 to complete the connection with the called line. If the called subscriber is in ofce I, the terminals selected by switch IS2 are associated with a twoway trunk circuit TG2 and the impulses created by the dialing of the succeeding digits of the called subscribers number are repeated through trunk circuits T04, TG3, TC2 and TCI to control the operation of an incoming selector ISI and a connector CI in olce I to complete the connec- On a call from the master ollce to -a line in otllce I or to a line in oilce .2, the connection is under the solev control of the calling operator.

Detailed description The opera-tions involved in the establishment of connections over trunks TI and T2 will now be explained in detail. i

Assume that the subscriber at station AI originates a call to the master omce, that a line nder LFI connects the calling line'to the first selector FSI, that the subscriber dials a digi-t which directs the selectorSI to the group of trunks leading to dial oilice 2 and that lthe twoway trunk circuit TCI, shown in detail in Fig. 2, is seized for use on the call. The selector FSI is represented in Fig. 2 by a set of four brushes and the single set Vof terminals'to which the trunk cir'- cuit TCI is connected. Uponseizureof trunk circuit TCI, the connection from the calling line is extended through the two upper Ibrushes of selector FSI, conductors II and I2, outer back contacts of relay '23, left windings of repeating coil RCI, inner back contacts of relay 23, to the windings of relay I1. Rel-ay I1, which is .thereupon operatively energized, closes a circuit for'opera-ting relay IIB. Relay I6 connects `ground potential to test conductor I 3 which is multipled to the test terminals of trunk TCI in the lbanks oi' all selectors having access to this trunk. The ground potential thus connected to the test terminal of selector FSI holds this selector and the associated line ilnde'r LFI under the controloi the calling subscriber and prevents the seizure of trunk TCI by any other selector having access thereto.

The subscribers lines to which 'the line finder `LFI has access maybe entitled to different classes of service and the trunk circuit TCI is arranged to transmit either one of two class-of-service tones, to the answering operator Vin the master oice in case a call is originated from a line in the one or the other of these two classes. If the call is from a line in one of these two classes, the line-finder and selector circuits LFI and FSI connect ground through brush I to completely oper" ate rel-ay 21in the trunk circuit TCI. If the call is from a line in the other of these two classes, the line-ilnder and selector circuits LFI and FSI connect ground through a resistor to .brush :I0 to partially operate relay 21,' in which case only the upper contac-t of relay 21 is actuated. In either case, relay 21 closesa circuit for operating relay 25.. Relay 25 closes a circuit for operating the start relay 29 of the .tone interrupter 30.

The aforementioned connection of ground'potential through the fron-t contact of relay I6 to conductor I3 also effects the operation of relay I9.

With relays I1 rand I9 operated,the windings of'y relay I are disconnected from the conductors 3| and 32 of `trunk TI and a bridge comprising the low-resistance, lower winding of relay I8 is connected across conductors 3| and 32 to effect the operation of marginal relay 50 of trunk circuit.v

TC2 at the other end of trunk TI. The circuit for operating relay 5I) is traced from battery .through fuse F3, upper winding of relay 50, inner'` upper back contact of relay 49, upper lef-t winding of repeating coil RC2, outer 4upper .back cony tact of relay 49, upper back contact of relay 40, conductor 32, upper back contact of relay 26, upper right winding of repeating 'coil RCI, inner fron-t contact of relay I9, lower front contact of relay I1, lower winding of relay I3, outer front cont-act of relay I9, lower right winding of repeating coil RCI, inner lower back contact of relay 26,

conductor 3 I, lower back contact of relay 40, outer lower back contact of relay 49, lower left winding of repeating coil RC2, inner lower back contact of relay 49, through the lower winding of relay 50 to ground. Relay I8 of trunk circuit TCI does,

not operate since the relayv is polarized and the current through its lower winding is not in the Relay V5Iv closes a circuit for is closed across conductors 88 and 66 ofthe rotary selector circuit RS. If al1 of the trunksv connected to the bank of selector RS 4are busy, relay 60 is operated due to the release of the all-trunks- `busy relay 99 shown in Fig. 5. In this case, re-

l'lay 60 prevents the operation of the line relay 6I,

13 is connected to ground andl the stepping magnet 10 is actuated in a circuit which includes the interrupter contactof magnet 10, the'upper front contact of relay 6I, 'and brush '13.'v The brushes are thereby advanced'by theoperation of magnet 10, step by step, until`brushf13 vengages the test terminal of an idle trunk, whereupon cut-01T relay 62 'is operated in' a Ylcircuitfrom battery through the winding and interrupter contact of magnet 10, winding of relay 62 to ground at the lower front contact of relay 6I. The magnet 10 is marginal and does not operate in series with relay 62. Relay 62 disconnects relay 6| from conductors 55 and 56 and connects these conductors to brushes 1 I and 12, thus completing a circuit for operating line relay89 of the seized trunk circuit TC3. 'I'his circuit is traced from battery through the right winding of relay 89, inner upper back contact of relay 81, upper left winding of repeating coil RC 3, upper back contacts of relays 81 and 80, conductor 18, terminal and brush 12, upper front contact of relay 62, conductor 56, thence through the bridge comprising the low-resistance winding of relay 42, conductor 55, lower front contact of relay 62, brush 1I and terminal, conductor 11, lower back contacts of relays 80 and 81, lower left winding of repeating coil RC3, inner lower back contact of relay 81, through the left winding of relay 89 to ground. Relay 89 closes circuits for operating relays 84 and 90. Relay 84 connects'groundto test conductor 19 to hold cut-off relay 62 and rotary selector circuit RS after relay 6| releases and to guard the trunk circuit TC3 against seizure by any other rotary selector or by any of the twomotion directively controlled selectors such as F S2. Relay 90 closes a circuit for operating the motor start relay MS of the tone and ringing interrupter and relay 84 closes a path from a code ringing conductor through condenser 9| to conductor 18 to transmit ringing tone to the calling subscriber as an indication that an operator in the master oilice is being signaled. Relay .90

' interchanges the connections between the windings of relay 86 and conductors 91 and 98 of tnmk T2 leading to the master oilice thereby effecting though the windings of relays`86 and v|00 are Aconnected in a normally closed circuit over trunk the currentis not strong enough to operate III relay IOI closes a circuit for operating relay |02; and relay |02 closes a circuit for operating relay |03. Relay IOI also closes a circuit for lighting answering lamp |I3. Relay |08 closes a circuit for lighting busy lamp II4 and 'connects a busy potential, through resistor I I6 and the lower winding of relay |06, to the sleeve of Jack I I2.

When an operator answers the call by inserting the plug of a cord in Jack II2, a circuit is closed from battery in the operators position circuit through the sleeve conductor of the answering cord and jack I|2 and the upper winding of relay |05. Reference may be had to the patent application of R. E. King et al. Serial No. 243,347, filed December l, 1938, for a disclosure of cord and position circuits arranged for cooperation with the trunk circuit TC4. Relay |06 operates. closing a circuit for operating relay |06. Relay |06 closes a circuit for operating relay |01; closes a short circuit around the high-resistance, upper winding of relay |00, thereby causing the operation of relay 86 of trunk circuit TG3; and opens the circuit through the winding of relay I0| to release this relay and extinguish lamp |I3. Relay |01 closes a holding circuit for relay |02 and completes a talking connection between the right windings of repeating coil RC 4 and the line conductors of jack I'I2. Y

The aforementioned operation of marginal relay 86, when the `call is answered, closes a circuit for operating relay 80. Relay 80' disconnects condenser 9| from conductor 18, to end the transmission of ringing tone to the calling subscriber, and interchanges the connections between the windings of relay 88 and the conductors 11 and 18 so as to reverse the current through the lower winding of polarized relay 42 of trunk circuit TG2 and thereby cause the operation of relay 42. Relay 42 closes a circuit for operating relay 40. Relay 40 interchanges the connections between the windings of relay 50 and conductors 3| and 32 of trunk TI thereby reversing the current through the lower winding of polarized relay I8 of trunk TCI. Relay I8 operates, closing a circuit for operating relay 24. Relay 24 closes a circuit for holding relay I6 and opens the circuit through relay 25. After relay 24 operates and before relay 25 releases, the upper left winding of repeating coil RCI is connected to condenser 28 and a tone is transmitted therethrough to the answering operator to indicate the class of the calling line, one class of service tone being transmitted if relay 21 is only partially operated, another class of service tone being transmitted if relay 21 is completely operated, and a third class of service being indicated by the transmission of no tone if relay 21 is neither partially nor completely operated.

The talking connection between the calling I line and the operator in the master oilce includes the repeating coils RCI, RC2, RC3 and RC4. 'I'he connection is under the joint control of the calling subscriber and the answering operator. When the calling subscriber releases the connection, relay I1 releases, thereby opening the short circuit around the upper, high-resistance winding of relay I8 to cause the release of relay 50 of trunk circuit TC2.. Relay 50 opens the short circuit around the upper, high-resistance winding oi' relay 42 thereby causingthe release ofy relay 89fof trunk circuit TC3. The release of relay 89 causes the release of relay 90 therebyrestoring the current over trunk T2 to itsnormal direction to eiect the release of relay |00 of trunk circuit TC4 in the master ofllce. When relay |00 releases. it connects ground to the lower, low-resistance winding of relay |08 thereby causing the lighting of a supervisory lamp in the operators-cord circuit as a disconnect signal.

When the operator disconnects, relay |05 releases. The release oi' relay |05 causes the release of relay |06 and the release of relay |08 causes the release of relay |01. Relay |0| reoperates if relay |00 has not released. 'I'he release oi' relay |06 opens the short circuit around the upper, high-resistance winding of relay |00 thereby causing the release of relay 86 of trunk circuit TO3. The release of relay 86 causes the release of relay 80. When relay 80 releases, the current through conductors 11 and 18 is restored to normal direction to eilect the release of relay 42 of trunk circuit TC2. The release of relay 42 causes the release of relay 40. When relay releases, the current through conductors 3| and 32 is restored to normal direction to eifect the release of relay I8 of trunk circuit TCI. The release of relay I8 causes the release of relay 24.

When lboth the subscriber and the operator have released the connection, the release of both relays |1 and 24 of trunk circuit TCI causes the 19 to effect the release of relay 88 of rotary selector RS. The release of relays'lil and |01 causes the release of relays |02 and |08 thereby circuit for operating relay 06. Relay |06 closesv a short circuit around the upper winding'of-rela'y |00 to eilect the operation of relay 86 oftrunlr" circuit TO3, closes a circuit for operatingy relay |01, andconnects ground to the lower 'winding of relayA 05 to effect 'the lightingofa super visory lamp in the operators cord circuit.; lRela |01 closes a circuit for operating relay |02, connects the winding of relay |08 through the .upper right winding of repeating coill RC4 to the tip of jack |12, and connects ground at the innert:4 lower front contact of relay |06 throughresistori5 and the lower right winding of coil -R`C4'j to the ring ofjack 2. Relay |02 closes acircuitforropcrcnng relay los. Relay los 'lights-the busy lamp H4. The operation of relay 86 V`closes a circuit, including back contacts of relaysl 8,4k

and 90, through the winding of relay 81. Relay v'81 operates, thereby connecting its winding 'toi sleeve conductor 95 of the associated incoming e selector IS2 and extending groundpotentlal to conductor 19 to`mark the trunk circuit TG3 as busy in the bank of'selectors FS2 and RS. `Relay 81 closes a bridge,comprising` the windingv of relay 88 andres'istor 92v in parallel, across conductors 93 and 94 of selector IS2 to prepare the selector forvoperating by dialjimpulses. The selector IS2 connects a holding ground potential to sleeve conductor 95 in usual manner to hold relay 81.

'dllcborsz91 and Th Relay ,86. "of trunkgcircuit TC3 is alternately released and reoperatedv by the. dial impulses v transmitted over- :conductors 91 and 98 by the v` [contacts kof; relayi 4||0.1;When relay 86l releases I' inresponseto'theiirstimpulse of a train, it opensv the .bridge aero s .l conductors 93. and 94, and

. .closesa cir'cult'lncluding .the inner lower. back Vof relay 86 closes abridg'e comprising resistor 98 When the callingoperator operates thedi'al key in the position circuit, battery is connected upper conductor ot Jack H2, outer upper front through the tip conductor of the operators cord. Y n

contact of relay |01, upper right winding of re- 5 -peating coil RC4, through the winding of relay |08 to ground. Relay |08 operates, closing a circuit for operating relay |08 and connecting the tip of jack ||2 to repeating coil RC4 independent oi.' the contact of relay |01. Relay |08 -closes a circuit for holding relay |02 operated' |08 and |08, andopens the, circuit through the winding of relay. |01. ,'Relay |81 releases, disconnecting the ring. of Jack ||2vflrom condenser |04. Relay m connects ground to the of relay ||0, thereby short-circuiting the upper winding of relay toprepare relay |.|0 for re' operated, the bridge comprising the'lower winding of relay |00 is disconnected from trunkv T2 and the impulse repeating contacts of relay |0 are connected across the conductors 91 and.

98. When the operator dials the number of a 30 l y vsponse= to dial impulses. With relays ||0 'and ,A

called subscriber, relay ||0 is alternately released is completed, the release of the operators dialers.

ke'y disconnects battery from `the tip of the cord,l releasing relay .|08'. Relay |08 opens the connection betweex'frepeating coil RC4 and the tip of jack H2, short-circuits the contacts of relay ||0, andv releases relays |09 and i lease of relay |09 disconnects the winding of relay ||0 from the ring of jack ||2 and o ens the holding circuit of relay |02. The `release" oi relay disconnects the contacts of relay "H0, from trunk AT2; and reestablishes thebridge'com'- prising the winding of relay |00 across the con-v een.'l relay |01'l and comple--` y nnectionnbetween jack H2.. RC4`.jl Relay v|01 recloses the f. 'l'filiff10,21andfrelayflliirefl vlrates.Since elay'vzl03 is lslow in releasing'itf mains perad, n'1ri-ngv the-'transfer from'` dialing-to talkingjconnections..

een@wennfifarid.;cn-,upperfrom Contact of y relay 8 1 for operatingthev slowfto-release/freY- lay 85.j-.With.relay85 operated,-the reoperation across conductors 93nar'idl 94 in place of the The vre 4() ereleasle of relay `|I|| also 1 bridge comprising thewindin'g of relay 88. Relay .Y `V

85 remains operated while'each trainof impulses, Y

i's being repeated by relayj86. 'I'he release of re-fA 1. e

. A* of relay 88 across conductors V9,3antl94j forrelay after each vvdigit:'reconn'ects. the winding ceiving 'supervisory signals.

vThe ilrst train'jof impulses repeated by relays ||0 and 86 control theoperationof the incoming.

selector IS2 to selecta corresponding group oi 75 l:ss

y 33 and 34 of incoming selector ISI.

conductor 34 of selector ISI.

trunks. The selector thereupon operates automatically to select an idle trunk in the selected group. On a call to dial ofllce I, seizure of trunk circuit T02 extends the connection from trunk circuit TC3 through conductors 93 and 94, conductors 51 and 58, back contacts of relays 54 and 53, right windings of repeating coil RC2, inner back contacts of relay 53, to the windings oi relay 43. Relay 43 operates, closing a circuit for operating relay 52. Relay 52 closes a circuit for energizing the upper, polarizing winding of relay 54; closes a circuit for operating relay '49; and connectsl ground to conductor 59 to prevent seizure of trunk circuit TG2. by any other selector, to hold the selector IS2, and to hold, relay 81 of trunk circuit TCI. With relays 43 and 49 operated, a bridge is closed across trunk TI to operate relay I5 of trunk circuit TCI in oilice I. This bridge is traced from conductor 3| through the inner lower front contact of relay 43, inner lower back contact of relay 44l outer lower front contact of relay 49, lower left winding of repeating coil RC2, inner lower iront contact of relay 49, resistor 48,"lower winding'of relay 54, outer lower back contact of relay A44, inner upper front contact oi relay 49, upperv left winding of repeating coil RC2, outer upper front contact of relay 49, upper back contact of relay 44, and the upper front contact of relay 43 to conductor 32. Relay 43 is alternately released'and reoperated in response to the dial impulses repeated by relays |I0 and 88 upon the dialing of the succeeding digits of the called subscribers number. When relay 43 releasesin response to the iirst impulse of each train, it closes a circuit for operating relay 44 and, being slowin releasing, relay 44 remains operated until all of the impulses in the train have been received by relay 43. Relay 44 disconnects the bridge comprising the lower winding of relay 54 from conductors 3| and 32. When relay 43 reoperates at 'the end of the first impulse, conductor 3| is connected through a lower front contact of relay 4,4 and resistor 46 to battery and conductor 32 is connected through an upper front contact of relay' 44 and resistor to ground. The windings of relay I5 of trunk circuit TCI are thereby energized in separate grounded circuits over conductors 3| and 32 during the transmission of impulses. When the last impulse of each train has been received, relay 44 releases to reconnect the lower winding of relay 54 across trunk TI. Relay 43 is thus effective to repeat the received impulses to relay I5 of trunk circuit TCI.

The operation of relay I5 of trunk circuit TCI, upon seizure of trunk circuit TC2 by selector IS2 as above described, closes a circuit for operating I4. Relay I4 connects ground potential to conductor I3 to mark trunk circuit TCI busy in the bank of selector FSI, closes a circuit for operatively energizing relay 23 and closes a circuit for energizing the lower polarizing winding of relay 26. Relay 23 vcloses a bridge across conductors This bridge is traced from conductor 33 through the back contact of relay 2|, outer lower front contact of relay 23, lower left winding of repeating coil RCI, inner lower front contact'of relay 23, upper winding of relay 26, inner upper front contact of relay 23, upper left winding of repeating coil RCI, outer upper front contact of relay 23, and

through the outer front contact of relay I5 to 'I'he selector ISI is thereby prepared for response to dial impulses in usual manner. Relay I5 is alternately recuit TCI.

Y 5 leased andreoperated in response to dial impulses repeated by relays IIO, 88 and 43. When relay I5 releases in response to the first impulse of each train. it closes a circuit for operating relay 2| and, belngslow in releasing, relay 2| remains operated luntil all of the impulses in the train have been received.` When relay I5 reoperates at the end of the first impulse, the loop across conductors 33 and 34 includes resistor 22 and the iront contact of relay 2| instead oi' the bridge comprising the winding of relay 26. I

'I'he ilrst train of impulses received and repeated by relay I5 is effective to directively operate selector ISI to select a group of connector switches. Selector ISI is then operated automatically to select an idle connector in the selected group and to extend the connection from conductors 33 and 34 to this connector. The impulses created by the dialing of the last two digits of the called subscribers number are repeated by relays 0, 86, 43 and |5 to control the operation of the connector to complete connection with the called line in usual manner. If the line is idle, the called station is signaled. If the call is answered the connector functions in well-known manner to reverse the direction of current through conductors 33 and 34 and the upper winding of relay 26 thereby eecting the operation of this relay. Relay 26 thereupon interchanges the connections between the windings of relay I5 and conductors 3l and 32 to reverse the current through the lower winding of relay 54 of trunk circuit TC2. Relay 54 is thereby operated and in turn effects the reversalof current through conductors 51 and 58, 93 and 94 and the winding of relay 88 of trunk circuit TC3. Relay 88 operates, closing a circuit for operating relay 90. Relay 90 effects the reversal of current through conductors 91 and 98 to operate relay |00 of trunk circuit TC4. The operation of relay |00 disconnects ground from the lower winding of relay |05 to extinguish the supervisory lamp in the operators cord circuit.

The talking connection between the operator and the called subscriber includes the repeating coils RC4, RC3, RC2 and RCI. I'he connection is under the sole control of the operator.

If the called subscriber restores the telephone to normal before the operator disconnects, the connector restores the direction of the current through conductors 33 and 34 to successively release relays 26,- 54, 88, 90 and I 00 and thus cause the lighting o! the supervisory lamp in the operators cord circuit. When the operator disconnects, relays |05, |06, |01, |02 and |03 are released. 'Ihe release of relay |06 opens the short circuit around the upper winding of relay |00 thereby causing the release of marginal relay 86 in trunk circuit TC3. The release of relay 86 opens the operating circuit of relay 81 and opens the bridge across conductors 93 and 94 thereby causing the release of relay 43 of trunk circuit TC2. When relay 43 releases, it causes the release of relay 52. Relay 52 releases ,relay 49 and disconnects the busy ground from conductor 59 thereby effecting the return of the selector IS2 to normal in usual and well-Jknown manner and the release of relay 81 of trunk circuit TC3. 'I'he aforementioned release of relay 43 also opens the Abridge across conductors 3| and 32 thereby releasing relay I5 of trunk cir- Therelease of relay I5 causes the release oi' relays I4 and 23 thereby opening the bridge across conductors 33 and 34 to release the line relay of the connector switch. If at that time the called subscriber has failed to hang up,

the connector switch and the selector ISI are.

held operated. When the called subscriber hangs up, the connector and incoming selector switches are restored to normal in usual and well-known manner, l

The trunk circuit TO3 may be seized' by a selector FS! on a call to the master omce from a subscriber in oillce 2 in which case the trunk' circuit functions in similar manner to that hereinbefore described on a call from omce I to the master office except that relays 8! and 8 funcv to prevent seizure of trunk circuit TG3 by any selectors such as RS and vFS2. An alarm call will thereupon be initiated automatically to the master oillce to indicate a trouble in the dial oilice.

'Ihe normally-operated make-busy relay 4I of trunk circuit TC! releases if the fuse F4 op- 'erates' thereby connecting ground to conductor 59 to guard this trunk circuit against seizure by any selector having access thereto. The lower contacts of relay 4I close a short circuit across conductors 3l and 3! thereby causing the operation of relay 50 of trunk circuit TC! and the operation oi.' relay I5 of trunk circuit TCI. Relay 50 effects the extension of a trouble call to the master oillce. Relay I5 closes a circuit for operating relay I4. Relay I4 connects ground to conductor I3 to guard against seizure of trunk circuit TCI by any selector in oice I.

'I'he normally-operated make-busy'relay 20 of trunlk circuit TCI releases if the fuse F! operates thereby connecting ground to conductor I3 to guard the trunk TCI against seizure in oilicel and closes a short circuit across the inner ends of the right windings of 1 repeating coil RCI thereby operating relay I5 of `trunk circuit TCI and operating relay 50 of trunk circuit TC!. Relay I5 performs no useful function at this time but relay 50 closes the circuit for operating relay 5I and effects the extension of a trouble call to the, master oilice. Relay 5I vconnects ground to conductor 59 to guard against seizure of trunk circuit TC! by any selector having access thereto.

A separate fuse is provided for each of relays I5 and 50 ofl trunk circuits TCI and TC! to insure the release of the make-busy relays and 4I if the fuse associated therewith operates. I f the same fuse was used to connect relay I5 to batteryas is used to connect relay!0 to battery, the operation of this fuse would not insure the release of relay 20 because this vrelay would then be connected in series with the lower winding of relay I5, conductor 3! of trunk TI and the upper winding of relay 50, to battery. By

-providing a separate fuse for relay I5, such a holding circuit for relay 20 is eliminated.- vFor a like reason a separate fuse is provided for relay 5I! in trunk circuit TC!. If fuse FI of selector TCI or fuse F3 of trunk TC! operates, the trunk will not be guarded against seizure but since these fuses are each individual to one relay, there is smaller probability of operation oi'- either of these fuses.

The foregoing description of the operation re- 5 sulting fromi operation of the fuse F! ottrunk v circuit TCI or from the operation oi.' the fuse F4 of trunk circuit TC! presumes that the trunk T I is not in use when the fuse operation occurs. Obviously, however, the operation of the fuse F! or F4 may occur at the same time that seizure y of the trunk is being eilected in either oillce o r' after seizure has occurred.

I-t is particularly to be noted lthat the provision. of a separate fuse for the line relay of l each of trunk circuits TCI and TC! and the provision of contacts on the make-busy relay in each of these trunk circuits, to simulate seizure if the circuit becomes inoperative^due to the blowing of the fuse F! or the fuse F4, is effective `l0 to guard the trunk TI against seizure at either end whenever such a fuse operation occurs. This f is-particularly important in case the fuse F4 of trunk circuit TC! operates because if the trunk circuit Tcl were not then guarded against seiz-` 2l ure, a calling subscriber in unattended office `I would b e unable to make a master ofilce call during times of light traillc if the trunk circuit TCI were rst choice inthe banks oi' all of the selectors throughwhich the calling subscribers 0 line may be connected to a trunk to the tandem dial omce. y

Although the invention has been described in detail in its application to trunk circuits arranged for loop signaling, it is, oi.' course, applicable to 35 systems and trunk circuits arranged for composite signaling, simplex signaling or any other type of signaling.

What -is claimed is:

1. In a telephone system oilce and a second oilice, a two-way trunk bel tween said oillces and automatic means in each of said ofIices for I'seizing said trunk on a call to the other of said ofces, means in said rst oiiice operatively responsive to. seizure of Vsaid f trunk in said second ofIlce forv marking the trunk busy, and means in said second. ofce for automatically simulating seizure of said trunk in said second oice.

2. In a telephone system comprising a first l office and a second oillce, a two-way trunk between said oilices and automatic means in each. of said oiilces for seizing said trunk on a call to the other of said ofilces, means in said rst oilice operatively responsive to seizure oi.' said 5 -trunk in said second oilice for marking the trunk office having access to said second trunk circuit,

means lin said first trunk circuit operative upon. seizure of said second trunk circuitby a selector vfor preventing seizure of said -rst trunk circuit by a selector, and means automatically effective if said second trunk circuit becomes inoperative for simulating seizure of said second trunk circuit.

4. In a multiofiice telephone system, a trunk,"

comprising a first 40 between two ofiices, a iirst two-way trunk circuit, a second two-way trunk circuit, said iirst trunk circuit terminating said trunk in one of said ofces and said second trunk circuit terminating said trunk in the other of said oflices, selector switches in the one oilice having access to said rst trunk circuit, selector switches in the other oiiice having access to said second trunk circuit, means in said ilrst trunk circuit operative upon seizure of said second trunk circuit by a selector for preventing seizure of said iirst trunk circuit by a selector, means in said second trunk circuit operative upon seizure of said first trunk circuit by a selector for preventing seizure of said second trunk circuit by a selector, and means in each of said trunk circuits automatically eiective if the trunk circuit becomes inoperative for simulating seizure by a selector having access thereto.

5. In a telephone system, a rst oiiice, a second office, a two-way trunk between said oflices, selectors in said ilrst oillce, selectors in said second oillce, each of said selectors being operable to select said trunk on an outgoing call, means in each of said oiices operatively responsive to seizure oi.' the trunk by a selector in the other ofdce for marking the trunk busy to selectors in the called oiiice, means in each of said oilices operatively responsive to seizure by a selector in the same oiiice for marking the trunk busy to other selectors in the same oilice, and means comprising a make-busy relay in each of said oflices for automatically marking the trunk busy in the same oilice and for simulating seizure of the trunk by a selector in the same office so as to effect a busy marking of the trunk in the other oilice. K

6. In a telephone system comprising a trunk and an associated two-way trunk circuit, selectors for seizing said trunk circuit on outgoing calls, means responsive to seizure of said trunk circuit by a selector for closing a bridge across the conductors oi' said trunk and i'or marking said trunk circuit busy to the other selectors, and a make-busy relay automatically operative ii said trunk circuit becomes unserviceable for marking said trunk circuit busy to all of said selectors and for closing a bridge across the conductors of said trunk.

7. In a telephone system comprising a trunk and an associated two-way trunk circuit, selectors for seizing said trunk circuit on outgoing calls, means responsive to seizure of said trunk circuit by a selector for sending a seizure signal over said trunk and for marking said trunk circuit busy to the other selectors, a fuse for supplying electrical energy to said trunk circuit, and a relay effective upon the blowing of said fuse to send a seizure signal over said trunk and to mark said trunk circuit busy to said selectors.

8. In a telephone system comprising a trunk and an associated two-way trunk circuit, selectors for seizing said trunk circuit on outgoing calls, means responsive to seizure oi' said trunk circuit by a selector for sending a seizure signal over said trunk and for marking said trunk circuit busy to the other selectors, a relay in said trunk circuit for receiving a seizure signal incoming over said trunk, a fuse individual to said relay for supplying electrical energy thereto, another fuse, 'said other fuse being individual to said trunk circuit for supplying electrical energy thereto, and means comprising a relay effective upon the blowing of the last-mentioned fuse to transmit a seizure signal over said trunk and to mark said trunk vcircuit busy to said selectors.

9. In a multiofce telephone system comprising an interofdce trunk and' an associated two-Way trunk circuit at each end of said trunk, selectors for seizing said trunk circuits on outgoing calls. means in each of said circuits operatively responsive to seizure by a selector on an outgoing call for sending a seizure signal over the trunk and for marking the trunk circuit busy to other selectors, a relay in each of said circuits operatively responsive to a seizure signal incoming over the trunk for marking the circuit busy to selectors having access thereto, fuses individual to each of said circuits for supplying electrical energy thereto, one of the fuses for each circuit being individual to said relay, and relay means in each of said circuits eilective upon the blowing of a fuse individual thereto for marking the trunk busy to selectors having access thereto and i'or sending a seizure signal over the trunk.

10. In a telephone system comprising a first ofce and a sec `ncl ofiice, a two-way trunk between said oflices, automatic means in each of.

said oilices for seizing said trunk on a call to the other fsaid oiilces, means in said iirst oiilce operatively responsive to seizure of said trunk in said second oiice for marking the trunk busy, and means in said second oiiice for automatically simulating seizure of said trunk to cause the operation of said busy marking means in said first oice.

1l. In a telephone system comprising a first oiice and a. second office, a two-way trunk between said oiiices, automatic means in each of said offices for seizing said trunk on a call to the other of said oiiices, means in each of said ofiices operatively responsive to seizure of said trunk in the other oi.' saidfoillces for marking the trunk busy, and means in each of said oices for automatically simulating seizure of said trunk to cause the operation 'or the busy marking means in the other of ,saidoiilces 12. In a telephone system comprising a first ofiice and a second olce, a two-way trunk between said oilices, .automatic means in, each of said offices for seizing said trunk on a call to the other of said oiiices, means in each of said oiices operatively responsive to seizure of said trunk in the other of said ofiices for marking the trunk busy, and means in each of said omces for automatically simulating seizure of said trunk to cause the operation of the busy marking means in the other of said oiiices and for simultaneously marking said trunk busy in the ofiice in which seizure is simulated.

RUSSELL B. BUCHANAN. CHARLES D. KOECHLENG. 

